In conventional digitizer tablets, a pointing device which may be a stylus, mouse, or other form of moveable element including a transducer is employed to receive signals from a tablet grid, or the grid is employed to receive signals from a pointing device transducer. In certain grid geometries, it is necessary to provide a reference that enables the determination of the absolute phase of the received signals in order to enable determination of the transducer location.
As an example of one such grid geometry, commonly-assigned U.S. Pat. No. 5,051,545 (the '545 patent), the contents of which are incorporated herein by reference, describes a position determining apparatus and conductor structures or grid layouts therefor. The conductors are arranged for each axis of the grid in a pattern such that signals obtained for that axis may be processed to provide binary numbers in a Gray-type code. Code is produced which is unique to a separate small region of the active area over which the coil center of an electromagnetic transducer is located. The small region for a given axis corresponds to the space between two immediately adjacent, active conductor portions for that axis. Small regions in each axis therefore define the coarse location of the coil center in that axis. The '380 patent describes a modification of the structure described in the '545 patent.
Inherent in the design of the digitizers described in the '545 patent is a requirement that the "phase" of the signal from each wire must be determined. If the transducer is coupled to the system via a cord, the controller in the system transmits a signal to the transducer (or grid) and detects the received signal from the grid (or transducer). In this type of system, the necessary information is present to determine whether the received signal is in-phase or out-of-phase with the transmitted signal. The digitizer uses a coding of "0" and "1" to indicate respectively "in" phase and "out" of phase signal states for each wire, and constructs a binary number from the induced signals which it then uses to determine where the transducer is located on the surface of the grid. This encoded binary number is unique at each wire space or coarse location.
A cordless transducer for use in a digitizer generally includes an independent oscillator, and so additional means must be provided to enable determination of the phase of this oscillation, so as to be able to construct the appropriate "0" and "1" codings for the wires. The concept of self-synchronization is described in copending application, Ser. No. 891,768, particularly as a means for providing a cordless digitizer with excellent noise immunity characteristics. In one of its preferred implementations in a cursor-driven system, a sync signal is derived from signals induced in a conductor of one grid while a conductor of the other grid is sampled to derive a location signal. This implementation assumes either a known relationship between the sync signal and the sampled location signal, or that the relationship is irrelevant to the position location program.
When the phase relationship between the transmitter and receiver is not known and is important to the position-location determination, as in the system of the '545 patent, it is still possible to determine the relative phases between the signals on the different wires, but it is not possible to determine the absolute phase of the signal in any wire. For example, it is not possible to determine the difference between the signal sequence "010" and its inverse "101". Hence, it is not possible to determine unambiguously whether the transducer is over one of several possible coarse locations in the above type of grid layout.
The occurence of inverse codes, in a digitizer of the type disclosed in the '545 patent, is demonstrated by reference to the following Table I, for a digitizer having a plurality of parallel wires interconnected to form three conductors, and separated by spaces S.sub.1 -S.sub.7. In this table it is seen that a number of the codes (in the second column) representing the spaces in the first column, have binary inverses of other ,codes (in the third column) in the same set.
TABLE I ______________________________________ SPACE CODE (Binary Inverse of Code) ______________________________________ S.sub.1 110 (001) S.sub.2 100 (011) S.sub.3 101 (010) S.sub.4 001 (110) S.sub.5 000 (111) S.sub.6 010 (101) S.sub.7 011 (100) ______________________________________
Thus, space S.sub.1 is the inverse of space S.sub.4, denoted as S.sub.1 =S.sub.4 ; S.sub.2 =S.sub.7 ; S.sub.3 =S.sub.6 ; S4=S.sub.1 ; only S.sub.5 does not have a corresponding inverse code.
If the phase relationship between the signals of the tablet and the transducer is not known, it is not possible to determine whether, for example, the transducer is in space S.sub.6 or S.sub.3 in this grid layout.